The present invention relates generally to micromechanical devices and more particularly to damped micromechanical devices.
Micromechanical devices have heretofore been provided, and include sensors such as accelerometers, angular rate sensors and gyroscopes and optical devices such as optical switches, scanners, interferometers and tunable filters. Each of such devices includes a moving structure supported by flexural elements and is thus a spring mass system having one or more mechanical resonant modes. These modal frequencies are typically estimated through the use of finite element analysis. A mechanical quality factor or Q, which is a measure of the damping associated with the motion of the part, can be associated with each of these resonant modes.
For micromechanical devices fabricated in materials such as silicon, silicon dioxide, silicon nitride, or metals such as aluminum or nickel, the inherent damping of the structural material itself is extremely low. For example, electrostatic microactuators manufactured using deep reactive ion etched (DRIE) techniques often have comb gaps on the order of ten microns and thus do not provide damping in air that is sufficient for using such microactuators as positionable actuators. As a result, such devices typically have measurements of the mechanical quality factor Q in a vacuum that are typically greater than 5,000 and are potentially susceptible to external vibration or shock, especially from disturbances closely matching the frequency of one of the mechanical resonant modes of the device. It is thus important to control the damping of micromechanical devices.
Although viscous damping of micromechanical devices occurs from the dissipation of energy resulting from the motion of fluid, such as air or liquid, in which the device resides, attempts to control the damping of such devices have been limited. For devices which operate at or near a mechanical resonance, such as some vibrational gyroscopes, it has been desirable to maximize the mechanical quality factor Q of the system by devising methods to package the devices in vacuum, thereby reducing the viscous damping due to air. Papers describing the effects of primarily air damping on a variety of micromechanical devices include: xe2x80x9cViscous Energy Dissipation in Laterally Oscillating Planar Microstructures: A Theoretical and Experimental Studyxe2x80x9d, by Y.-H. Cho, et. al., 1993 Proceedings IEEE Micro Electro Mechanical Systems Workshop, Feb, 1993, pp. 93-98, and xe2x80x9cEvaluation of Energy Dissipation Mechanisms in Vibrational Microstructuresxe2x80x9d, by H. Hosaka, et. al., 1994 Proceedings IEEE Micro Electro Mechanical Systems Workshop, February 1994, pp. 193-195. Neither of these papers, however, contains recommendations for modifying the geometry or structure to optimize the damping of a device.
Some micromechanical devices, such as sensors, have relatively limited mechanical motion and can thus be controlled by including structures with small gaps, typically on the micron scale, in the device. In this technique, called squeeze-film damping, motion of the part causes such a gap to open and close, resulting in a fluid such as air flowing in and out of the gap. One of the many papers describing the use of holes through a structure to modify the squeeze-film effect is xe2x80x9cCircuit Simulation Model of Gas Damping in Microstructures with Nontrivial Geometriesxe2x80x9d, by T. Veijola, et. al., Proceedings of the 9th Int. Conference on Solid-State Sensors and Actuators, Stockholm, June, 1995, pp. 36-39. Unfortunately, squeeze-film damping is not generally suitable for devices having greater than a few microns of motion.
A limited amount of work has been done with linear accelerometers by packaging them in a viscous liquid, such as a silicone oil, to minimize xe2x80x9cringingxe2x80x9d caused by the response of the accelerometer to shock. The practical issues involved with using fluids other than air to control or adjust damping in micromechanical devices have been discussed. See, for example, xe2x80x9cA Batch-Fabricated Silicon Accelerometerxe2x80x9d, by Lynn Roylance, IEEE Trans. Elec. Dev., Vol. ED-26, December, 1979, pp1911-1917. See also International Application No. PCT/N092/00085 having International Publication No. WO 92/20096 by T. Kvisteroy et al. entitled xe2x80x9cArrangement for Encasing a Functional Device, and a Process for the Production of the Samexe2x80x9d. Neither of these publications, however, discuss the damping of actuators.
The energy loss and thus the mechanical quality factor Q of micromachined cantilever beams and other mechanical resonators have heretofore been studied. See, for example, xe2x80x9cDominated Energy Dissipation in Ultrathin Single Crystal Silicon Cantilever: Surface Lossxe2x80x9d, by J. Yang, et. al., 13th Annual International Conference on Micro Electro Mechanical Systems (MEMS 2000), Miyazaki, January 2000, pp. 235-240, which discusses the influence of various atomic layers such as silicon dioxide and absorbates on the surface of the cantilevers on the mechanical quality factor Q of the cantilevers. See also U.S. Pat. No. 5,659,418 entitled xe2x80x9cStructure for Membrane Damping in a Micromechanical Modulatorxe2x80x9d, which discloses controlling the damping of a device with mechanical transmission lines that couple the vibration from the modulator structure to the damping region of the device. Unfortunately, neither of these publications discuss controlling or modifying the mechanical quality factor Q of an actuator device.
As can be seen, none of the foregoing techniques has been used with actuators, and specifically with electrostatic actuators.
In general, it is an object of the present invention to provide a micromechanical device which is damped so as to control the resonant mode of the microactuator contained therein.
Another object of the invention is to provide a microactuator of the above character in which a material is adhered to a flexural member of the microactuator to damp the microactuator at such resonant mode.
Another object of the invention is to provide a microactuator of the above character in which the material is an elastomeric material.
Another object of the invention is to provide a microactuator of the above character in which the material is adhered to the flexural member after the manufacture of the microactuator.
Another object of the invention is to provide a microactuator of the above character in which the material is adhered to the flexural member during the manufacture of the microactuator.
Another object of the invention is to provide a micromechanical device of the above character in which the material is introduced into an etched recess during the manufacture of the flexural member.
The present invention provides a damped micromechanical device comprising a substrate, a movable structure overlying the substrate and a flexural member having a first end portion coupled to the substrate and a second end portion coupled to the movable structure. The movable structure is movable at a resonant frequency between first and second positions relative to the substrate. A damping material is adhered to at least a portion of the flexural member for damping the movement of the movable structure at the resonant frequency. A method for making the micromechanical device is provided.